Currie F9 Rear End - Burly Backend

Currie Enterprises' F9 Custom Rears Can Handle Some Serious Power.

Since building gratuitous amounts of horsepower is getting easier and easier every day, it's not a far-out thought to have a street car putting out 700 to 1,000hp. With that level of power on tap, all the stuff behind the engine gets put though the proverbial wringer. We are going to run into that exact situation with a car our editor is building.

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When asked about his new ride, he responded, "The car is a '72 Nova SS that I bought sans motor. It was set up for a 454, but I'll be putting in a 509ci Dart big-block and a lightweight TCI front clip. It was mini-tubbed before I bought it, but the previous owner kept the 10-bolt in it, which won't last five minutes behind the 669hp Rat, hence the need for a stronger rearend. It should be a fun street/strip car that'll (I hope) run high 10s on street tires, then maybe 9s when we hit it with some nitrous."

As you can tell from his plan, the car will be flogged on a regular basis and will see the business end of a nitrous bottle at some point. To keep all the power under control and transfer it to the tarmac, the vehicle will need a rear that can take the abuse of a lead-footed editor. Enter Currie Enterprises and the company's F9 fabricated 9-inch. This built-to-order differential can be built in multiple power-handling capacities to suit your specific needs and budget.

On Currie's recommendation, we decided on an F9 with 3-inch axle tubes, 35-spline axles, nodular iron 9+ Race Gear Case, 9+ Big Bearing pinion support, 9+ nodular iron yoke for a 1350 U-joint, and large bearing housing ends (Torino-style). Everything we just listed is actually made by Currie in house, well except for the casting/forging process. They make it all right down to the wheel studs and other hardware.

The only outside parts we needed to order were the Motive Gear 3.70:1 gear set and the Detroit TrueTrac helical gear differential. The gears will work great with the 27.8-inch diameter back tires and the TrueTrac will distribute the power to both wheels while still being streetable.

Currie recommended the TrueTrac for our F9 application. The TrueTrac differential from Eaton distributes power to both wheels without using wearable parts like clutch plates and springs found in traditional limited-slip differentials. The TruTrac uses a set of helical gears that mesh with side gears to allow one wheel to spin under light load for better street drivability. Once some torque is applied the gears lock up inside the diff and send power down both axles.

Currie uses Motive Gears in all the rearends it builds. "We use Motive Gears because we found they are high quality and provide quiet operation. Plus, Motive offers a tremendous range of ratios so we feel confident in getting the customer exactly what they want," said Brian Shephard from Currie.

Currie can install traditional street/strip gears like in our rearend or they can put in a soft, lightweight AX Motive Gear for full competition use.

We invaded the Currie building and followed the process of how the company builds this rear from basically scratch. Once the rearend was completed, it would be able to handle 1,000-1,200hp and live a nice long life under the Nova. Now if our editor Jim gets a little power hungry and upgrades the engine past 1,200hp, we'll need to have an even stouter rearend built by Currie, maybe a chrome-moly housing with 4-inch tubes and 40-spline axles.

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Currie F9 Rear End - Burly Backend

Here is the center of the F9 rearend that Currie designed and built. The housing body is built out of 3/16-inch Hi-Form steel, while the third member face is 3/8-inch thick Hi-Form steel. Inside you will find gussets to provide more strength to the third member face and bulkheads to add more support for the axle tubes. The F9 provides 3 inches of tube engagement as well, way more than any stock 9-inch housing.

Here is a look at the gussets used to strengthen the third member face.

Like we stated in the opening text, Currie builds the F9 from scratch, so the axle tubes start as 3-inch DOM tubing that is rough cut with a band saw. For an even stronger rearend Currie offers chrome-moly tubes and housings.

Then the tubes are chucked in a lathe where they are cut to the perfect size. The inner diameter is opened up so the large bearing ends will slide in.

One key feature to adding durability to the complete unit is how Currie indexes the axle tubes into the housing to prevent them from twisting.

Here you can see where the flat part that was just cut in the tube indexes into the F9 housing. Once welded, it will be nearly impossible to twist the axle tubes.

Another area that is unique to the F9 is this inner bulkhead that provides another area of support to the axle tubes.

Because it was getting pretty difficult to find bearing ends off of original rearends, Currie has new ones cast off site before they are machined. These are the large bearing or Torino-style ends which use standard seals and bearings so you can get parts from your local parts supplier, or Currie of course.

With the housing in a fixture and a mock-up third member and axle in place, the tubes and bearing ends are trued and set to customer specs.

Once everything checks out, the tubes and ends are tacked in place.

Now the housing is moved to the welding room so all the components can be fused into one beefy unit. Here, the inside end of the axle tubes are welded to the bulkhead.

At this point any accessories are attached as well like these leaf spring perches. These are 3/16-inch thick and will work with our multi-leaf pack. Currie also offers pads for mono spring applications as well.

Now that the housing is all welded together, it is put into another fixture and trued up with a press. With the amount of welding that was performed, things are bound to move a little. After straightening, the housing is put into a Steelabrator Airless shot machine. This removes all scale and other contaminants, inside and out, for a super clean product.

The third member will be assembled using Currie's 9+ Race Gear case, 9+ nodular iron yoke for a 1350 U-joint, and 9+ Big Bearing pinion support. Also in the mix is the 3.70:1 gearset from Motive Gear and the Detroit TrueTrac diff. The 9+ case is a completely redesigned piece that has all kinds of improvements built into it to provide more strength and support to the ring and pinion.

Step one is to join the ring gear to the TrueTrac differential with the new hardware. The ring gear is a tight fit so it is pulled on using the bolts and an impact gun.

The pinion support is temporarily assembled with a different yoke and bolts, then installed into the gear case. The new yoke will be installed after the gear lash is set up.

Now the diff assembly is lowered into the third member and held in with the stronger designed billet steel caps.

Gear lash is set with a dial gauge. Currie builds so many of these that the technician got it right on the first try.

Since the gear engagement was spot-on, the screw-in side adjusters were locked in place with these bolt-on tabs. This will prevent the adjusters from rotating and allowing the gears to come out of adjustment.

After another round of checks the caps are torqued down.

Now the temporary yoke and bolts are replaced with new parts. Again, Currie not only manufactures the housing and what not, but it also has stronger hardware made specifically for the rearends.

After a quick coat of paint to prevent rust, the third member is bolted to the housing. A new gasket and a small bead of silicone are used to create a leak-free seal.

The axles start out as raw 1541 forgings that have been induction heat-treated. They are quickly trimmed close to our spec with a chop saw.

After the cut end is perfectly sized and the end chamfered on a lathe, they are placed in a grinding machine that cuts in 35 splines.

The next CNC machine drills the lug pattern 5 on 4 3/4...

...with 1/2-inch studs in our case.

A very large press is used to install the studs and the bearing assemblies. The bearings used are Timken set-20 tapered roller bearings.

The axle assemblies are slipped into the fresh case and held in place with stronger Currie-made retainer plates.

With that, the rearend was ready to ship out to Jim so he could add his Wilwood brakes and fill it with 84-140w non-synthetic gear oil. This point is very important as Currie has done extensive research and found the synthetic fluids don't provide enough protection under extreme loads. In fact, using synthetic fluid will void any expressed or implied warranty on Currie products. We will revisit this rearend when Jim gets it under his Nova and takes it to the track.

If you are not willing to spend boat loads of money on a repair shop, then we have a 1966 Chevy Nova SS that needs a rear quarter panel replacement and we will be walking you through the whole process. - Super Chevy Magazine » Read More